In the preparation of photomechanical printing plates black-and-white halftone colour separation prints are used as photomasks in the production of printing plates for printing with subtractive colour printing inks. Inks of the subtractive colours cyan, magenta and yellow are used, as well as a certain amount of black ink applied by black printer.
The amount and distribution pattern of any one of the printing inks used in conjunction with the different printing plates for multicolour printing depends on the dot size distribution in the printing plates, which distribution in turn depends on the dot size distribution in the black-and-white halftone colour separation used as a photomask in the photo-exposure for making the plate.
In some cases the available halftone separations need enlargement on films or printing plates, e.g. for use in the production of large size posters, advertisement panels, large format publications or needs reduction or enlargement for adapting the format between different publications.
The production of halftone separations at reduced or enlarged size proceeds usually by projection exposure in a graphic art camera. In practice the halftone separations require frequently adaptation in size in the range of 70 to 600% with respect to the original.
Halftone reproduction by optical projection exposure in an enlargement camera is associated with a problem in that a correct reproduction of the whole tonal scale of the screen dot pattern of the halftone original used in the projection exposure is impossible. For example, an exposure dose resulting in a correct reproduction of halftone areas having a 50% dot value results in an overexposure in the areas having a % dot value higher than 50, and results in an underexposure in the areas having a % dot value lower than 50.